The expression of different Fc receptors on discrete cell types allows a combinatorial linkage of the exquisitely sensitive antibody response to cell-mediated immune mechanisms, including cytotoxic, degranulatory and phagocytic reactions. We have obtained diffraction quality crystals for both the IgE-receptor and the IgE-Fc ligand. Elucidation of these structures will provide a new of this important link between antibody specificity and cellular immune responses. Enveloped viruses have evolved intriguing mechanisms for entry into their host cells. The paramyxovirus F protein mediates the fusion of the virus envelope and target cell membrane. Two F-protein fragments assemble into a "post-fusion"structure, which might arise after a triggered conformational change in the intact F protein. The crystals of this F-protein "core" will provide new insights into the fusion mechanisms of the paramyxovirus family.